How Many Blocks Do You Need for a Retaining Wall?
Enter your wall length and height to get blocks, cap blocks, and gravel to order.
Last updated: May 31, 2026
Wall Dimensions
Wall Order Summary
132
blocks to order
Cap Blocks20 blocks
Wall Area40 sq ft
Layout20 columns × 6 rows
Extra for Waste+12 blocks (10%)
Base & Backfill Gravel
Total Gravel2.15 cu yd
Weight~3.01 tons
20 ft × 2 ft · Standard (12" × 4") · 10% waste included
Worked Examples
Small Garden Bed Wall — 10×2 ft
Standard 12"×4" block · 10% waste
- Columns: ceil(10 ft × 12 ÷ 12") = 10
- Rows: ceil(2 ft × 12 ÷ 4") = 6
- Base blocks: 10 × 6 = 60 × 1.10 = 66 blocks to order
- Cap blocks: 10 blocks
Standard Driveway Wall — 20×2 ft
Standard 12"×4" block · 10% waste
- Columns: 20 · Rows: 6
- Base blocks: 120 × 1.10 = 132 blocks to order
- Cap blocks: 20 blocks
- Base + backfill gravel: ~2.15 cu yd
Hillside Wall — 40×4 ft
Standard 12"×4" block · 10% waste
- Columns: 40 · Rows: 12
- Base blocks: 480 × 1.10 = 528 blocks to order
- Cap blocks: 40 blocks
- Note: Walls over 3 ft may require geogrid reinforcement and permit
Block Size Guide
| Size | Coverage | Best For |
|---|---|---|
| Standard 12"×4" | 1 sq ft per block | Most DIY walls under 3 ft — easy to carry, widely available |
| Small 8"×4" | 0.67 sq ft per block | Curves, corners, and smaller projects |
| Large 18"×6" | 1.5 sq ft per block | Taller walls — fewer blocks, heavier (60–80 lbs each) |
| Jumbo 24"×6" | 2 sq ft per block | Large contractor projects — requires equipment to place |
Wall Height Guide
| Height | DIY Friendly | Notes |
|---|---|---|
| Under 2 ft | ✅ Easy | Garden beds, landscape edging — no permit needed |
| 2–3 ft | ✅ Standard DIY | Most common DIY height — typically no permit required |
| 3–4 ft | ⚠️ Check local codes | May require permit; geogrid reinforcement recommended |
| Over 4 ft | 🔴 Engineer required | Structural engineer and permit required in most areas |
Frequently Asked Questions
For a 20 ft × 2 ft wall using standard 12"×4" blocks: 20 columns × 6 rows = 120 base blocks, plus 10% waste = 132 blocks. Add 20 cap blocks for the top row. Use this calculator for your exact dimensions and block size.
Standard 12"×4" interlocking blocks are the most popular DIY choice — one person can carry them and they don't require mortar. For walls under 3 ft, standard blocks work well. For taller walls (3–6 ft), larger 18"×6" blocks reduce the total count. See the Block Size Guide below.
A retaining wall needs two gravel layers: a compacted base (6 inches deep, twice the block depth wide) for stability, and drainage backfill (12 inches behind the wall). Enable base materials in Advanced settings and the calculator handles both automatically.
For a DIY dry-stacked interlocking block wall: up to 3 ft without engineering review in most areas. Walls over 3 ft often require a permit and geogrid reinforcement. Above 4 ft, consult a structural engineer — soil pressure increases significantly with height.
Base (field) blocks form the main body of the wall — they're typically hollow or slotted for stacking. Cap blocks are solid, flat-topped blocks placed on the final row for a finished appearance. You need one cap block per column (= wall length ÷ block width).